The answer is that PIM is not enabled on all interfaces between the BSR and the other routers. This is the most likely cause because BSR messages are flooded hop-by-hop using PIM as the transport protocol, meaning every intermediate router must have PIM enabled on its interfaces to forward the Bootstrap messages to downstream neighbors. If PIM is missing on any link in the path, the BSR advertisements will be dropped, and routers beyond that point will never learn the RP set, even though the candidate RPs are correctly advertising to the BSR. On the ENCOR 350-401 exam, this scenario tests your understanding that BSR relies on PIM adjacency for propagation, not unicast routing, which is a common trap—engineers often check unicast reachability first. A helpful memory tip is "PIM must be on every hop for BSR to drop," reminding you that BSR messages are PIM control packets and cannot traverse non-PIM interfaces.
CCNP IP Multicast Practice Question
This 350-401 practice question tests your understanding of ip multicast. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network engineer is configuring PIM sparse mode in a network that uses a Bootstrap Router (BSR) for RP discovery. The engineer has configured a candidate BSR and candidate RPs. However, some routers in the network are not learning the RP set. The engineer checks the BSR and sees that it is receiving candidate RP advertisements, but the BSR messages are not being forwarded to all routers. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
PIM is not enabled on all interfaces between the BSR and the other routers.
BSR messages are flooded hop-by-hop using PIM. If PIM is not enabled on all interfaces between the BSR and the other routers, the BSR messages will not be forwarded, preventing RP discovery.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
PIM is not enabled on all interfaces between the BSR and the other routers.
Why this is correct
Correct because BSR messages rely on PIM to flood; without PIM on intermediate interfaces, the messages are dropped.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Incorrect because priority affects BSR election, not the flooding of BSR messages.
✗
The candidate RPs are not in the same OSPF area as the BSR.
Why it's wrong here
Incorrect because BSR messages are flooded regardless of OSPF areas, as long as PIM is enabled.
✗
The BSR is not configured as a candidate RP.
Why it's wrong here
Incorrect because the BSR does not need to be an RP; its role is to distribute RP information.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-401 OSPF questions on adjacency and route selection.
IP Multicast — This question tests IP Multicast — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: PIM is not enabled on all interfaces between the BSR and the other routers. — BSR messages are flooded hop-by-hop using PIM. If PIM is not enabled on all interfaces between the BSR and the other routers, the BSR messages will not be forwarded, preventing RP discovery.
What should I do if I get this 350-401 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-401 OSPF questions on adjacency and route selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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